For more than half a century, the cathode ray tube (CRT) has been the principal electronic device for displaying visual information. The widespread usage of the CRT may be ascribed to the remarkable quality of its display characteristics in the realms of color, brightness, contrast and resolution. One major feature of the CRT permitting these qualities to be realized is the use of a luminescent phosphor coating on a transparent faceplate.
Conventional CRT's, however, have the disadvantage that they require significant physical depth, i.e., space behind the actual display surface, making them bulky and cumbersome. They are fragile and, due in part to their large vacuum volume, can be dangerous if broken. Furthermore, these devices consume significant amounts of power.
The advent of portable computers has created intense demand for displays which are light-weight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional CRT, there has been significant interest in efforts to provide satisfactory flat panel displays having comparable or even superior display characteristics, e.g., brightness, resolution, versatility in display, power consumption, etc. These efforts, while producing flat panel displays that are useful for some applications, have not produced a display that can compare to a conventional CRT.
Currently, liquid crystal displays are used almost universally for laptop and notebook computers. In comparison to a CRT, these displays provide poor contrast, only a limited range of viewing angles is possible, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color screens tend to be far more costly than CRT's of equal screen size.
As a result of the drawbacks of liquid crystal display technology, thin film field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employ a matrix-addressable array of pointed, thin-film microtips providing field emission of electrons in combination with an anode comprising a phosphor-luminescent screen.
The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays appear to be promising.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,857,161, "Process for the Production of a Display Means by Cathodoluminescence Excited by Field Emission," issued 15 Aug. 1989, to M. Borel et al.; U.S. Pat. No. 4,857,799, "Matrix-Addressed Flat Panel Display," issued 15 Aug. 1989, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to M. Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to R. Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to J.-F. Clerc. These patents are incorporated by reference into the present application.
The Spindt et al. ('799) patent discloses a field emission flat panel display having a glass substrate on which are arranged a matrix of conductors. In one direction of the matrix, conductive columns comprising the cathode electrodes support the microtips. In the other direction, above the column conductors, perforated conductive rows comprise the gate electrodes. The row and column conductors are separated by an insulating layer having holes permitting the passage of the microtips, each intersection of a row and column corresponding to a pixel.
The prior art references teach the use of various materials as the conductors comprising the cathode and gate electrodes. Among the materials suggested as the cathode conductor are indium oxide, tin dioxide, aluminum, antimony-doped or fluorine-doped tin oxide, tin-doped indium oxide (ITO), and niobium, citing their properties of good electrical conductivity and good adhesion to the substrate and the insulating layer. For the gate conductor, the prior art references recommend niobium, tantalum, aluminum, molybdenum, chromium, antimony-doped or fluorine-doped tin oxide, and ITO, citing their properties of good adhesion to the insulating layer and chemical resistance to the products used to form the microtips. Among these materials, niobium is the conductor most commonly cited for use as the cathode and gate electrodes.
While niobium performs adequately as the electrode material in field emission devices, it does present certain disadvantages. For example, it is not a material which is commonly used in ordinary semiconductor fabrication processes, it is relatively expensive, and, most significantly, it is not a good bonding material for interconnects or integrated circuits. It is therefore desirable to provide a material for use in a field emission device as the metallization layers which form the gate and cathode electrodes, the integrated circuit (IC) mount pads and the lead interconnects, which material is cheaper than niobium, is more commonly used in the semiconductor industry, and provides improved bonding over niobium to IC's and interconnects.